Evaluate Several Potential Evapotranspiration Methods for Regional Use in Tabriz, Iran

Accurate evapotranspiration estimates are needed to determine the crop water requirements for the purpose of irrigation scheduling. Numerous methods have been developed for evapotranspiration estimation out of which some techniques have been developed partly in response to availability of data. The Penman-Monteith equation standardized by the Food and Agriculture Organization (FAO56-PM) is accepted as a standard method to estimate ETo. Therefore, the other methods are evaluated by FAO56-PM method results. Nine methods in three groups, which are temperature based methods (Thornthwaite (TH), Hargreaves (HA) and Blaney-Criddle (BC)), solar based method (Priestley-Taylor (PT), Makkink (MA) , Rs (Ir- Rs) and Rn (Ir- Rn) based methods), and combined methods (Penman (PE) and Penman-FAO24 (PE24)), were used to evaluate with FAO56-PM method to choose the best method for using in research and projects. Performance analysis for the estimated values using climatic data for 7 years (Jan 2003 – Dec 2009) and validation of methods using 2 years climatic data (Jan 2010Dec 2011) were made. The root mean square error (RMSE) was calculated on a monthly basis. The estimated values by the mentioned methods were all correlated with FAO56-PM having RMSE values were 67.89, 49.45, 36.22, 61.35, 63.98, 58.28, 58.26, 5.48, 23.19 mm/month, respectively. The difference among the nine ETo methods before calibration showed a wide range of variation across the research period about 712 mm/yr. As a result of using the calibrated constant values in the equations, all nine methods are able to estimate monthly values perfectly. The RMSE of methods after validation were 37.16, 21.50,16.67, 27.82, 17.50, 17.70, 32.15, 2.62, 3.70 mm/month, respectively. Investigation shows the closer slope to 1 and intercept to 0 for PE method. However, the worst amounts of slope, intercept, R 2 and RMSE before calibration were belong to MA, BC, IR-Rn and TH, respectively. The best output after calibration was belong to the BC method without considering the combined methods, and the TH method had worst results. Although these revised methods increase the accuracy of estimates, FAO56-PM method is preferred if it is applicable due to the complexity of its input parameters.